Ethylenically unsaturated compounds, and in particular acrylate derivatives, can be polymerized by irradiation with ultraviolet light of wavelength between 200 and 450 nanometers (nm) in the presence of a bimolecular photoinitiating system. Typically, the photoinitiating system includes (1) a diaryl ketone photoinitiator and (2) a coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor. The coinitiators or synergists are typically alcohols, tertiary amines or ethers which have available hydrogens attached to a carbon adjacent to a heteroatom.
Examples of commercially available diaryl ketone photoinitiators useful in bimolecular photoinitiator systems include benzophenone, 2,4-dimethylbenzophenone, isopropylthioxanthone, and 2,4-diethylthioxanthone. The UV absorption spectrum for these individual photoinitiators, however, do not match-up efficiently with the UV emission spectra of the standard commercially available mercury vapor bulbs.
Thus, commercially viable UV curing processes can require a relatively large amount of initiator and synergist incorporated into the formulation. This can lead to cured articles which contain high levels of residual photoinitiator and synergist, which in turn can result in decreased light fastness and lower resistance to oxidative degradation. In addition, the residual photoinitiator and synergist can be extracted or leach out of the cured article or migrate to the surface of the article. Many times the physical properties of the article are degraded by the presence of the residual photoinitiator and synergist.